The present invention generally relates to protein products, such as plant protein and specific animal protein having reduced off-flavor and processes for making these protein products with reduced off-flavor. In particular, the present invention relates to extraction processes for removing off-flavor precursors utilizing either supercritical carbon dioxide or supercritical carbon dioxide and an organic solvent in order to achieve the protein products with reduced off-flavor. The produced protein products are suitable for use in a number of food products.
In response to the results of recent research showing the negative effects of certain foods on health and nutrition, consumers are becoming more health conscious and monitoring their food intake more carefully. In particular, since animal products are the main dietary source of cholesterol and may contain high levels of saturated fats, health professionals have recommended that consumers significantly reduce their intake of red meats. As a substitute for red meats, many consumers are choosing non-red meat products such as plant protein products, dairy protein products and egg protein products.
It is well known that vegetable products, such as soy protein products, contain no cholesterol. For decades, nutritional studies have indicated that the inclusion of soy protein in the diet actually reduces serum cholesterol levels in people who are at risk. Further, the higher the cholesterol level, the more effective soy proteins are in lowering that level.
Despite all of the above advantages, it is well known that by supplementing foods with increased levels of dietary fiber and protein, taste can be seriously compromised. More particularly, protein sources, such as soy protein, can produce objectionable off-flavors in the finished products. For example, many consumers complain that high protein foods, like those supplemented with soy protein, taste grassy, beany, and bitter. Soy off-flavors may be responsible for most of the complaints with respect to the taste of soy-based products.
It is believed that the development of soy off-flavors is initiated when phospholipids and triglycerides undergo hydrolysis to yield polyunsaturated free fatty acids, which then react with molecular oxygen to form fatty acid hydroperoxides and other oxygenated lipid species. Both the hydrolysis and the oxidation can occur in enzyme-catalyzed and in non-enzyme-catalyzed reactions. The hydroperoxides then decompose into smaller molecules such as aldehydes and ketones and it is these small molecules that are responsible for the odor and flavor of vegetable oil-based products. In particular, Boatwright (U.S. Pat. No. 6,426,112), Boatwright et al., J. Food Sci. vol 66, page 1306 (2001), Boatwright et al., J. Food Sci. vol 65, page 819 (2000), Y. Feng, et al. (Aroma Active Compounds in Food, ACS Symposium Series 794, ed. G. R. Takeaka et al., page 251 (2001)), and A. Kobayashi et al. (J. Agric. Food Chem., vol. 43, page 2449 (1995)) have identified some of the most flavor active of these molecules in soy isolate and soymilk, which contribute to soy protein's unique flavor. Specifically, these molecules may include methanethiol, dimethyl trisulfide, 2-pentyl pyridine-(E,E) 2,4-nonadienal, (E,Z) 2,6-nonadienal, (E,E) 2,4-decadienal, (E,Z) 2,4 decadienal, acetophenone, hexanal, 1-octen-3-one, beta-damascenone, (E) 2-nonenal, (E) 4,5-epoxy-(E)-2-decenal, vanillin, maltol, 1-octen-3-ol, 2-pentyl furan, 2-heptanone, octanal, (E) 3-octen-2-one, 2-decanone, benzaldehyde, and 2,3-butanedione. Most of these flavor active volatiles are derived from oxidation of polyunsaturated lipids. The formation of these flavor molecules and their hydroperoxide precursors begins as soon as the bean is crushed and continues through the soy isolate manufacturing process. Traditional processing methods have not been completely successful in reducing the level of off-flavors and off-flavor precursors to an acceptable level in finished soy isolate or in foods to which it is added.
The conventional process for manufacturing soy protein isolate begins with the production of full fat flakes from soybeans, which are substantially defatted with hexane. This process typically removes more than 80% of the acid hydrolysable lipids in the flakes, as measured by AOAC Method 922.06, while leaving behind the majority of the phospholipids present. Soy protein is then extracted from the defatted flakes/flour with water and separated from the insoluble vegetable matter via centrifugation. The extracted protein is precipitated, washed, resuspended in water and spray dried as described, for example, in Hettiarachchy, et al., Soybeans: Chemistry, Technology, and Utilization, pp. 379-411, Aspen Publishers, (1997), which is incorporated herein by reference in its entirety.
These processes are unsuccessful in producing a soy protein with an acceptable flavor because the hexane is inefficient at removing all of the phospholipids and triglycerides that contain polyunsaturated fatty acids. Low levels of these off-flavor precursors, and some of the enzymes which act on them, remain after the hexane extraction. These components continue to generate off-flavors during the removal of hexane from the defatted flakes at elevated temperatures. The defatted flakes which serve as the source of the soy isolate thus typically contains about 2.8% to 5.0% of lipid (dry basis), which may be analyzed as acid hydrolysable fat, and about 1.0% phospholipids, which may be analyzed by conventional HPLC methods. It also contains appreciable quantities of the flavor-active volatiles that persist through the subsequent protein isolation steps to result in isolate with the familiar grassy and beany flavors. The extraction with hydrocarbon solvents, such as hexane, has the additional disadvantage of creating air pollution due to the inevitable leakage of solvent into the atmosphere.
As an alternative to traditional hexane extraction, food scientists have evaluated the use of supercritical carbon dioxide (CO2) to remove the lipids and the off-flavor molecules from soy products. This process offers the advantage of a non-polluting solvent whose residues, because of its low boiling point, are much easier to remove from the defatted flakes. Two general approaches that utilize supercritical carbon dioxide to reduce off-flavors in soy products have been suggested. The first approach, as disclosed by Friedrich in U.S. Pat. No. 4,493,854, employed supercritical carbon dioxide to isolate the oil present in soybean. Friedrich also converted the defatted flakes into two isolate samples which were claimed to have improved flavor as compared to commercial isolate prepared from hexane-defatted flakes. While the grassy and beany flavors were removed from the two isolates, their overall flavor scores (6.0, 7.1) were only slightly improved compared with a range of commercial isolates referred to by Friedrich in K. Warner et al, Cereal Chem. 60:102 (1983), which had an average flavor score of 6.1. It is believed that better flavor scores were not obtained for the samples derived from the CO2-extracted flours for several reasons. Firstly, very high temperatures (84-100° C.) were used during the extraction in order to maximize oil recovery, and these high temperatures may have induced off-flavor formation. Secondly, supercritical CO2 is a poor solvent for phospholipids, and the high phospholipids levels present in the full fat flake would be little changed after extraction. Even though the extracting solvent was removed at lower temperatures than hexane, it is likely that the high residual levels of phospholipids regenerated the off-flavor volatiles that may have been removed during the extraction step. Other polar lipids, such as lipid hydroperoxides and other oxygenated lipid species may have low solubility in supercritical CO2 and may remain to serve as sources of off-flavor aldehydes and ketones.
It may also be possible that CO2 is simply not a strong enough solvent to remove the off-flavored aldehydes and ketones that are known to bind tightly to the soy protein.
An alternative approach to improve the flavor of soy protein isolate is to remove the off-flavor molecules by extracting them with supercritical solvent, such as supercritical CO2, after the protein has been isolated from the flake. For example, P. Maheshwari, E. T. Ooi, and Z. L. Nikolov, J. Amer. Oil Chem. Soc., 72:1107 (1995) extracted soy isolate with supercritical CO2, liquid CO2, and a mixture of 95% supercritical CO2/5% ethanol. Although the extracted isolates had a lower intensity of beany odor and improved overall acceptability compared with the starting isolate, each still retained significant flavor scores for beany odor. Thus, for the same reason outlined above, it is probable that high concentrations of phospholipids and oxygenated lipid species remain in the extracted isolates and cause the residual beany flavor.
While the prior art has demonstrated that supercritical CO2 extraction may have an impact on the intensity of soy beany flavors, processes used to date have not been entirely satisfactory because they leave behind significant quantities of off-flavor precursors. These precursors quickly regenerate the beany off-flavors which a majority of consumers find to be unacceptable.
Many of the volatiles noted above that contribute to the flavor of fresh soy isolate gradually increase in concentration as the isolate ages during storage. This phenomenon increases the intensity of the off-flavor and makes the isolate less and less acceptable to consumers as it ages. Any process which decreases the rate at which these volatiles are formed will lead to an increase in the shelf life of the soy isolate.
Additionally, as a wet process conducted at benign temperatures and at pHs ranging from 4 to 9, soy isolate manufacturing is prone to microbial growth. Unless it is carefully controlled, the growth of these microorganisms in-process will lead to the production of off-flavors and pathogens. Accordingly, a zero tolerance of pathogenic organisms and a maximum acceptable level of total plate count are commonly specified for the defatted flakes/flour that are used as the raw material for the soy isolate process. The small but acceptable levels of non-pathogenic organisms usually present in the defatted flakes/flour inevitably lead to growth during the soy isolate manufacturing process. This growth is minimized by minimizing the time of exposure of the isolate to aqueous conditions during the process and by frequent cleaning of the manufacturing equipment. Both of these conditions restrict the flexibility of the manufacturing facility and increase the costs of its operation. Any process that lowered the total plate count on the incoming defatted flakes/flour would therefore improve the operating efficiency of the soy isolate process.
As is evident from the foregoing, a need exists in the industry for a defatted flakes/flour with a minimized microbiological load, for soy protein isolate having reduced off-flavors and a longer shelf life, and a process of making such a soy protein isolate. Additionally, it would be beneficial if the defatted flakes/flour have low levels of lipids. Further, it would be beneficial if this process could be versatile and work with different commercial soy starting materials.